Recent years, a digital camera and so on have been developed, thus a request has been increased such that a large number of images, each of which includes a large number of character images such as those on a white board used at a conference and on a time table, should be taken into photographs and stored, and later, character information and the like should be read out from the photograph information. Such a photograph image by a digital camera is a multi-valued image having depth. If image information of the multi-valued image is stored as it is and outputted according to needs, it is possible to read out the character information and the like from the photograph information of the multi-valued image.
However, the multi-valued image has a large amount of information, and storing the image information requires an enormous amount of memories. For a request to simply read a character, it is not preferable that the multi-valued image is stored in memories as it is, and it is desirable that this multi-valued image is binarized to be stored in the memories. Such binarization of a multi-valued image having a depth is required for not only the foregoing example but also various other applications. For example, the binarization is required for the case where an image is reduced in size to be stored, the case of performing pre-processing made for recognizing an object in an image such as a character, the case of obtaining a special image processing effect (such as a woodcut print image) and so on. As principal binarization methods, there have conventionally existed a simple binarization method, an edge emphasis binarization method, a binarization method designed to pursue photograph quality as much as possible by using a binary printer driver program and so on.
Moreover, in a gazette of Japanese Patent Laid-Open No. Sho 63-108470, in consideration of a defect that the sphere of characters or figures whose binarized images may be sufficient is stored as multi-valued data, a technology is disclosed, in which information for classifying spheres into respective spheres to be saved as binary data and multi-valued data is added to image data and stored, thus the image data is stored in a small volume. Furthermore, in a gazette of Japanese Patent Laid-Open No. Hei 5-225388, a technology is disclosed, in which each pixel is filtered depending on a change of image densities of pixels existing on a periphery thereof to smoothen and thin image data, thus pre-processing is executed for enhancing clearness of an image to perform character recognition and so on.
However, the simple binarization method as described above is hardly used actually, though it is very simple. For example, if in an image, there are dark and bright portions that are mixed with each other, it often occurs that a density of a character existing in the bright portion is brighter than a background in the dark portion. For this reason, with this simple binarization method, there remain unsolved problems that an outline of a character or an object in the bright portion or the like is faded and that an outline of a character or an object in the dark portion or the like turns solid black together with a background. Moreover, the edge emphasis binary method cannot deal with a character having, for example, a blurred outline, therefore this method cannot be used for character recognition and the like. Furthermore, the binarization method designed to pursue photograph quality as much as possible by using a binary printer driver program provides the most natural image exhibiting good quality when viewed as a photograph by a person, but this method is not suitable for character recognition and cannot be used for the purpose of reducing a data volume to a great extent. Furthermore, in the prior arts described in the gazettes of Japanese Patent Laid-Open No. Sho 63-108470 and Hei 5-225388, a calculation amount is enormous and image processing to a great extent is required. Accordingly, the image processing using the binarization methods cannot be achieved with a simple system constitution by these prior arts, and it is difficult to attain high-speed processing therewith.